1. Field of the Invention
The present invention relates to a gain setting method for an optical disk reproducing apparatus such as a compact disk player.
2. Description of the Related Art
Recently, a recording-type CD called a CD-RW that is capable of overwriting has been developed. Furthermore, a low-reflectance disk is also developed for a CD-R that is a recording-type CD incapable of overwriting. In a conventional optical disk reproducing apparatus disclosed in Japanese Published Patent Literature (Japanese Unexamined Patent Publication 2000-100059), a function of switching the gain of an amplifier in two stages, i.e. the gain of one time or four times relatively to the normal gain corresponding to such recording-type CDs, is incorporated. In the conventional optical disk reproducing apparatus, both the CD with high reflectance and the CD (CD-RW and the like) with low reflectance can be reproduced by switching the gain of the amplifier.
FIG. 7 is a flowchart of a conventional disk identifying method. In an example of the conventional gain setting procedure, gain switching processing at the time of identifying the disk, and two-stage gain setting (rough gain setting and precise gain setting) in accordance with the gain switching processing are performed.
First, the gain switching processing performed at the time of identifying the disk is described. A laser is set ON in step 121, and injecting signal is inserted in the tracking direction in step S122 to drive a readout device reciprocally in the tracking direction via a driver. In step 123, the maximum and minimum values of the focus error signal in that state are measured and the maximum value of the amplitude of the focus error signal is obtained from the difference thereof. Then, the measured maximum amplitude of the focus error signal is compared to a threshold value in step 124. When the maximum amplitude is larger than the threshold value, it is shifted to step 125. When the maximum amplitude is smaller than the threshold value, it is shifted to step 126.
In the step 125, the gain is set to keep as it is one-time (×1), the tracking gain is provisionally set also as one-time (×1), and the RF is set as one-time (×1) as well. Then, it is shifted to step 127. In the meantime, in the step 126, the gain is changed to four times (×4), the tracking gain is provisionally set also as four times (×4), and the RF is set as four times (×4) as well. Then, it is shifted to the step 127 where the disk is rotated. In step 128, injecting signal is inserted in the tracking direction to drive the readout device reciprocally in the tracking direction via the driver. In step 129, the maximum and minimum values of the tracking error signal in that state are measured and the maximum value of the amplitude of the tracking error signal is obtained from the difference thereof. Then, the measured maximum amplitude of the tracking error signal is compared to the threshold value in step 130. When the maximum amplitude is larger than the threshold value, it is shifted to step 131. When the maximum amplitude is smaller than the threshold value, it is shifted to step 132.
In the step 131, identification of the disk is ended while the gains set in the steps 121-130 are kept as it is in those conditions. Meanwhile, in the step 132, identification of the disk is ended after changing the gains.
Next, the rough gain setting will be described. The rough gain setting is performed respectively in the focus error and the tracking error. In step 133, the readout device is driven reciprocally in the focus error direction for the focus error and in the tracking error direction for the tracking error, respectively, and the amplitudes of the error signals are measured in step 134. In step 135, the injecting signal amplitude is determined based on the result of the measurements in the step 134, and setting of the rough gain is completed. Determination of the injecting signal amplitude may be set as the amplitude of the injecting signal that may be easily used for the precise gain control, e.g. the value that is ½ times the measured amplitude. Measurement of the amplitude of the error signals is carried out at the time of identifying the disk. However, it is measured again at the time of setting the rough gains, since there are cases where the gains are changed at the time of identifying the disk.
Next, the precise gain setting will be described. First, in step 137, injecting signal is generated based on the gains determined in the rough gain setting described above so as to operate the readout device. The amplitude of the error signals is measured in step 138, and the gains are determined in step 139. Thereby, the setting is completed.
Although there is switching at two-stages in accordance with the reflectance performed in the conventional disk reproducing apparatus and the gain setting method thereof, the gains are fixed at the ratio of the input resistance of the operational amplifier to the feedback resistance thereof. Therefore, the individual dispersions may be generated when mass-production is carried out as products, due to characteristic dispersions in the lasers, light-receiving elements and the like which constitute the readout device, and the characteristic dispersions in the resistances which constitute the operational amplifier. In addition, there also exist individual dispersions in the reflectance of the disks to be reproduced. As a result, dispersions are also generated in the levels of the signals inputted to an A/D converter at a latter stage of the amplifier, which results in generating dispersions in the error signals. However, when there are dispersions in the error signals, the servo accuracy for each of the focus and tracking signals is deteriorated so as to also generate dispersions in the RF signals. Normally, the dispersions in the RF signals generated in this manner are absorbed with an AGC or an equalizer at the latter stage. If the dispersions become so large that they cannot be absorbed, the reproducing capacity of the disk reproducing apparatus becomes deteriorated. Thus, it becomes impossible to perform fine reproduction, including the cases where there is a fingerprint or the like adhered to the surface of the disk.
Furthermore, in the servo LSI and the like of low years, there are such types that the precise gain control is performed on the error signals inputted from the amplifier through carrying out the operational processing by the digital filter after A/D conversion. In such a structure, the readout device is operated actively (this operation is referred to as active operation hereinafter), and the error signal in that state is measured to calculate the gain. However, it is necessary to perform two stages of gain calculating processing, i.e. the gain calculation that is performed in advance for determining the gain of the applying drive signal at the time of the above-described active operation, and the gain calculation performed based on the measurement of the error signal at the time of the active operation. For the control thereof, an extra ROM area is required in the controller.